MYOFIBRILS: CHARACTERISTICS, STRUCTURE, COMPOSITION, FUNCTIONS - BIOLOGY - 2025

The myofibrils are the structural units of muscle cells, also known as muscular fibers. They are very abundant, they are arranged in parallel and are embedded by the cytosol of these cells.

Striated muscle cells or fibers are very long cells, measuring up to 15 cm long and 10 to 100 μm in diameter. Its plasma membrane is known as sarcolemma and its cytosol as sarcoplasm.

Diagram of the muscular structure of a human being (Source: Deglr6328 ~ commonswiki, via Wikimedia Commons)

Within these cells there are, in addition to the myofibrils, multiple nuclei and mitochondria known as sarcosomes, as well as a prominent endoplasmic reticulum known as the sarcoplasmic reticulum.

Myofibrils are recognized as the "contractile elements" of muscles in vertebrate animals. They are composed of several types of proteins that are what give them the elastic and retractable characteristics. In addition, they occupy an important part of the sarcoplasm of the muscle fibers.

Differences between muscle fibers

There are two types of muscle fibers: striated and smooth fibers, each with an anatomical distribution and a specific function. Myofibrils are especially important and evident in the striated muscle fibers that make up skeletal muscle.

Striated fibers present a repetitive pattern of transverse bands when viewed under the microscope and are associated with skeletal muscles and part of the cardiac muscles.

Smooth fibers, by contrast, do not show the same pattern under the microscope and are found in the characteristic muscles of the vasculature and digestive system (and all viscera).

General characteristics

Myofibrils are made up of two types of contractile filaments (also known as myofilaments), which in turn are made up of the filamentous proteins myosin and actin, which will be described later.

Graphic representation of myofibrils in skeletal muscle (Source: Modified from BruceBlaus via Wikimedia Commons)

Different researchers have determined that the half-life of the contractile proteins of the myofibrils ranges from 5 days to 2 weeks, so that the muscle is a highly dynamic tissue, not only from the contractile point of view, but also from the synthesis and renewal point of view. of its structural elements.

The functional unit of each myofibril in muscle cells or fibers is called the sarcomere and is delimited by a region known as the "Z band or line", from where the actin myofilaments extend in parallel order.

Since the myofibrils occupy a substantial part of the sarcoplasm, these fibrous structures restrict the location of the nuclei of the cells to which they belong towards the periphery of the cells, close to the sarcolemma.

Some human pathologies are related to the displacement of the nuclei towards the interior of the myofibrillar bundles, and these are known as the centro-nuclear myopathies.

Formation of myofibrils or "myofibrillogenesis"

The first myofibrils are assembled during the development of embryonic skeletal muscle.

The proteins that make up sarcomeres (the functional units of myofibrils) are initially aligned from the ends and sides of “premiofibrils” that are composed of actin filaments and small portions of non-muscular myosin II and α-specific actin of muscle.

As this occurs, the genes encoding the cardiac and skeletal isoforms of α-actin are expressed in different proportions in muscle fibers. First the amount of cardiac isoform that is expressed is greater and then this changes towards the skeletal one.

After the formation of the premiofibrils, the nascent myofibrils assemble behind the zone of premiofibril formation and in these the muscle myosin II form is detected.

At this point, the myosin filaments align and complex with other specific myosin-binding proteins, which is also the case with actin filaments.

Structure and composition

As mentioned a moment ago, myofibrils are composed of contractile protein myofilaments: actin and myosin, which are also known as thin and thick myofilaments, respectively. These are visible under the light microscope.

- Thin myofilaments

The thin filaments of the myofibrils are made up of the protein actin in its filamentous form (actin F), which is a polymer of the globular form (actin G), which is smaller in size.

The filamentous strands of G-actin (F-actin) form a double strand that coils into a helix. Each of these monomer weighs more or less 40 kDa and is capable of binding myosin at particular sites.

These filaments are about 7 nm in diameter and run between two areas known as the I band and the A band. In the A band, these filaments are located around the thick filaments in a secondary hexagonal arrangement.

Specifically, each thin filament is symmetrically separated from three thick filaments, and each thick filament is surrounded by six thin filaments.

The thin and thick filaments interact with each other through “cross bridges” that protrude from the thick filaments and appear in the myofibril structure at regular intervals of distances close to 14 nm.

Schematic representation of the myofilaments that make up the myofibrils and their cross sections (Source: Kamran Maqsood 93 via Wikimedia Commons)

The actin filaments and other associated proteins extend beyond the "edges" of the Z lines and overlap the myosin filaments towards the center of each sarcomere.

- Thick myofilaments

The thick filaments are polymers of the myosin II protein (510 kDa each) and are delimited by regions known as "A bands".

Myosin myofilaments are approximately 16 nm long and are arranged in hexagonal arrangements (if a cross section of a myofibril is observed).

Each myosin II filament is made up of many packed myosin molecules, each consisting of two polypeptide chains that have a club-shaped region or "head" and that are arranged in "bundles" to form the filaments.

Both bundles are held across their ends in the center of each sarcomere, so that the "heads" of each myosin are directed toward the Z line, where the thin filaments are attached.

The myosin heads fulfill very important functions, since they have binding sites for ATP molecules and, in addition, during muscle contraction, they are able to form the cross-bridges to interact with the thin actin filaments.

- Associated proteins

Actin filaments are "anchored" or "fixed" to the plasma membrane of muscle fibers (sarcolemma) thanks to their interaction with another protein known as dystrophin.

In addition, there are two important actin-binding proteins known as troponin and tropomyosin that, together with actin filaments, form a protein complex. Both proteins are essential for the regulation of the interactions that take place between thin and thick filaments.

Tropomyosin is also a two-stranded, filamentous molecule that associates with actin helices specifically in the region of the grooves between the two strands. Troponin is a tripartite globular protein complex that is arranged in intervals on the actin filaments.

This last complex functions as a calcium-dependent "switch" that regulates the contraction processes of muscle fibers, which is why it is of utmost importance.

In the striated muscle of vertebrate animals, in addition, there are two other proteins that interact with the thick and thin filaments, known as titin and nebulin, respectively.

Nebulin has important functions in regulating the length of actin filaments, while titin participates in the support and anchoring of myosin filaments in a region of the sarcomere known as the M line.

Other proteins

There are other proteins that associate with thick myofilaments known as myosin-binding protein C and myomesin, which are responsible for the fixation of myosin filaments in the M line.

Features

Myofibrils have elementary implications for the movement capacity of vertebrate animals.

Since they are constituted by the fibrous and contractile protein complexes of the muscular apparatus, these are essential to execute the responses to the nervous stimuli that lead to movement and displacement (in skeletal striated muscles).

The indisputable dynamic properties of skeletal muscle, which comprises more than 40% of body weight, are conferred by myofibrils which, at the same time, possess between 50 and 70% of the proteins in the human body.

The myofibrils, as part of these muscles, participate in all its functions:

- Mechanical: to convert chemical energy into mechanical energy to generate force, maintain posture, produce movements, etc.

- Metabolic: since the muscle participates in the basal energy metabolism and serves as a storage site for fundamental substances such as amino acids and carbohydrates; it also contributes to the production of heat and to the consumption of energy and oxygen used during physical activities or sports exercises.

Since myofibrils are composed mainly of proteins, they represent a storage and release site for amino acids that contribute to the maintenance of blood glucose levels during fasting or starvation.

Also, the release of amino acids from these muscular structures is important from the point of view of the biosynthetic needs of other tissues such as the skin, brain, heart and other organs.

References

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